The short answer is that it's not worth anyone's time and to tap your phone for information of that low a value.
Phones can be tapped yes, chances are that those doing the tapping already have your information available. Even if they didn't, tapping phone lines is a huge effort and could only done with any amount of economy if all your calls could be transcripted automatically and then have the information scraped out. That's a lot of expensive technology there, just to get your credit card or SSN details. If a person was going to do it without that technology they'd be committing themselved to days of tedious work in the hopes of getting one piece of data. It simply isn't worth the effort to do when there's much easier ways of getting that information.
Now if the value of the information is higher it may be worth the effort. Say it's not SSN or Credit card details but information that may effect a stock's price, the value of commodities, government secrets, or give evidence of criminality. The value of the information is greater than the effort and cost it would take on a phone tap.
Even if your credit card and ssn details were valuable you are hampered by technology. Although there is technology that could encrypt your phone calls, it requires that both sides use it, and it is expensive. To make that work across all banks and people would require investment that is far above the actual value of the information it is protecting, which is why it hasn't been done in the past.